Various processes have been disclosed for the industrial dehydrogenation of ethylbenzene to styrene (Ullmanns Encyklopadie der technischen Chemie, 4th edition, Vol. 22, page 298). In practical use is the adiabatic process which is usually carried out in two radial-flow reactors which are connected in series, with intermediate heating or feeding in of steam, and which are operated adiabatically, and the isothermal process in which a tube bundle reactor with indirect heat transfer is used. Steam is additionally used in this case too.
It is also known that the space-time yield of the isothermal process in simple tube bundle reactors in which only the tubes are filled with catalyst can be improved by having an adiabatic catalyst layer upstream (DE-A-1 059 437) or downstream (DE-A-1 290 130). DE-A-34 43 118 also describes a combination of an upstream and a downstream reaction zone operated adiabatically with a tube bundle reactor.
However, the feeding-in of a large amount of steam which has been preheated to a high temperature to initiate an adiabatic reaction has the disadvantage that precisely the advantage of the isothermal process, namely the operation with a low steam/ethylbenzene ratio is abandoned and part of the possible space-time yield is dispensed with.